Soybeans have been processed into tofu (bean curd) or deep-fried tofu (deep-fried bean curd) and eaten as high-quality vegetable protein since early times. The soybeans contain water-soluble dietary fibers such as pectin and insoluble dietary fibers such as cellulose. The dietary fibers are classified into the water-soluble fibers and insoluble fibers, and the soybeans contain the insoluble dietary fibers in large amounts. Therefore, soybean fibers have been discarded as tofu refuse (bean curd refuse) in tofu manufacturing. The dietary fiber is a general term for indigestible components which are not digested with a human digestive enzyme, and in particular, the insoluble dietary fibers have been considered to be useless. In recent years, however, it has been suggested that the insoluble dietary fibers exert an effect of excreting food residues by absorbing water in a body to swell and stimulating the intestinal wall. The insoluble dietary fibers are considered to have an intestinal regulation effect necessary for modern eating habits. The tofu refuse, which is separated and discarded in a tofu manufacturing step, is the very insoluble fiber fraction in soybeans. Therefore, tofu industry participants have deeply desired that tofu containing insoluble soybean fibers is put into public circulation.
Therefore, soybean fiber-containing soy milk drink and tofu-like food have been developed actively. However, the soybean fiber-containing soy milk drink and tofu-like food have many problems to be solved in terms of quality, and have been marketed as only limited special products. In the late 1980's, instead of conventional methods involving pulverizing tofu refuse finely and blending the resultant in soy milk or tofu, there have been more frequently employed a method involving dehulling washed soybeans (Patent Document 1) and a method characterized in that fiber-containing soy milk drink or tofu-like food can be manufactured easily using finely pulverized soybean powder (Patent Document 2 and Patent Document 3), and tofu-like food has been sold actively in various regions. However, the soybeans contain 50 to 57% of linoleic acid and 5 to 9% of linolenic acid as unsaturated fatty acids. Therefore, autoxidation due to oxygen in air generated in a pulverization process or in storage of the powder and oxidation reactions of fatty acids due to a lipoxygenase contained in soybean seeds in the process of dissolving the soybeans in water are promoted to produce unpleasant odor (grassy smell) at a concentration significantly higher than that in usual tofu manufacturing. At the same time, oxidation reactions of soybean polyphenols are induced in dissolution of the powder, and hence unpleasant taste such as astringent taste is exhibited strongly. As mentioned above, conventional products each including soybean powder has critical disadvantages.
In order to solve such disadvantages, there is proposed a method of obtaining fiber-containing soy milk drink and tofu-like food by heating raw soybeans to thermally deactivate a lipoxygenase which is a lipid oxidase contained in soybean seeds in advance and pulverizing the soybeans finely (Patent Document 4). However, in the method, storage proteins contained in the soybean seeds are heat-denatured to decrease the water-soluble protein content in the powder because the raw soybeans are heated before pulverization. Therefore, in the case where fiber-containing tofu-like food is manufactured using the powder, it is very difficult to manufacture ideal tofu-like food because the gel strength of the protein is insufficient. In particular, as is the case with semi-aseptic type or aseptic type long life (LL) filled tofu, in the case where heat treatment is performed sufficiently in advance for pasteurization or sterilization of soybean fiber-containing soy milk, the gelling ability of soy milk is reduced to one-third of that in unheated one. Therefore, it becomes impossible to achieve gelation using only the tofu coagulant, and even if the soy milk is coagulated using a transglutaminase (TG) as a coagulation aid, the gelation depends on the coagulating ability of only the transglutaminase, resulting in a texture significantly different from tofu.
Fiber-containing soy milk and tofu-like food made from soybean powder have the following disadvantages in terms of texture as well. That is, the fiber-containing soy milk and tofu-like food are lack of smoothness required for conventional soy milk and tofu, and even if the powder particle size is adjusted to 100 micrometers or less, which is the minimum size of particles obtained commercially at present, uncomfortable feeling remains after eating because of the roughness of the powder particles. Therefore, it is very difficult to ingest fiber-containing soy milk or tofu in a required amount of about 100 to 200 g per meal, which is not acceptable to consumers in the actual circumstances.
In order to eliminate the roughness due to the insoluble fiber, it has been proposed that soy milk and tofu are manufactured by treating soybean powder with a fiber-degrading enzyme to obtain a soluble fraction and blending the resultant in original soy milk (Patent Document 5). However, in order to treat the soybean powder with the enzyme, it is necessary to perform enzymatic reactions for a long period of time around 50° C. which is an optimum reaction temperature for the enzyme. Therefore, it is necessary to find out how to suppress microorganism contamination in the step of reactions of the fiber-degrading enzyme added to a water dispersion of raw soybean powder. The soybeans are covered with soil-derived heat-resistant spore-forming bacteria, and the bacteria are transferred to soybean powder, which causes a very difficult problem in a countermeasure to the contaminant in the step. Further, the method requires a long-term enzymatic reaction time around 50° C., which promotes the action of a lipid oxidase derived from soybeans as well, inevitably resulting in deterioration of the flavor or the like. As mentioned above, in the methods proposed in the literature, it is very difficult to keep the quality of soy milk drink or a tofu-like product on a high level and to supply the drink or product stably at a low cost and a large scale.
Meanwhile, as a method of manufacturing fiber-containing soy milk drink or tofu-like food using soybean powder, there are proposed methods each involving dispersing powder in water and performing high-pressure homogenization treatment (for example, Patent Documents 6 and 7). However, in the proposed methods, the soybean powder is simply dispersed in water, and hence the action of an oxidase or the like contained in the soybean powder is caused. Even if emulsification treatment is performed in a short period of time, the emulsification action promotes enzymatic reactions, resulting in accumulation of substances causing unpleasant odor and unpleasant taste. Further, the high-pressure treatment increases the liquid temperature of the powder dispersion. For example, the product temperature increases by 8° C. at 25 MPa and by 10° C. at 50 MPa, and hence enzymatic actions are promoted with increase in the product temperature, resulting in further promotion of enzymatic oxidation.
Meanwhile, there is proposed a method involving using TG in combination with a tofu coagulant in manufacturing fiber-containing tofu using soybean powder (Patent Document 8). However, the proposed method includes dissolving soybean powder in water and heating the resultant dispersion substantially (according to Examples) at 100° C., and hence the gelation ability of the soybeans remains strongly. Even if coagulation thereof is performed with only the tofu coagulant, the tofu has roughness owing to soybean particles and has a lowered commercial value. Therefore, TG is used in combination with the coagulant, but in order to eliminate the roughness, a significant amount (0.1 to 0.3% by weight relative to the soybean powder dispersion) of TG should be used. Accordingly, the resultant tofu has extremely high elasticity which is completely different from the texture of tofu owing to a synergistic effect with the original gelation ability of soybeans. This is shown in a report on use of a tofu coagulant and TG in manufacturing fiber-containing tofu-like food using soybean powder (Non Patent Document 1), which describes that even if TG is used in combination, the resultant product has a rough texture and is inferior in smoothness compared with tofu containing no fibers, and that if the amount of TG added is increased to 0.15% or more, the resultant product has a pudding-like texture. That is, it has been recognized that, even if TG is used, it is difficult to manufacture fiber-containing tofu-like food having excellent texture.